Most if not all electronic devices require a DC voltage of appropriate level for proper operation. The DC voltage is typically derived from an AC power source, e.g. by plugging a power supply into a wall socket. The AC voltage available at the wall socket is converted to a DC bulk voltage by a full-wave rectifier diode bridge. The DC bulk voltage is further converted to a regulated DC output voltage by a switching power supply.
The switching power supply uses a transformer, or an inductor depending on the configuration, as an energy transfer element. For example, a flyback-type power supply has a power switching transistor coupled to one side of the primary winding of a transformer. The power transistor turns on and off as determined by a regulator circuit to alternately store energy in the magnetic field of the transformer and transfer the stored energy to the secondary winding. The secondary winding of the transformer develops an output voltage across a shunt capacitor coupled across the secondary winding as a function of the energy transfer. The voltage across the capacitor provides the DC output voltage of the switching power supply.
The DC output voltage increases and decreases with the applied load. An increasing load decreases the DC output voltage and a decreasing load increases the DC output voltage. The DC output voltage, or a representation thereof, is fed back to the regulator circuit to allow the switching power supply to compensate for load variation. As the load increases, the DC output voltage decreases causing the regulator to leave the power transistor on for a longer period of time to store more energy in the magnetic field. The additional energy is transferred to the secondary winding during the off time of the power transistor to supply the increased load and re-establish the DC output voltage. As the load decreases, the DC output voltage increases which causes the regulator to leave the power transistor on for a shorter period of time to store less energy in the magnetic field. The reduced energy transfer to the secondary winding during the off time of the power transistor causes the power supply to adjust to the decreased load and reduces the DC output voltage back to its steady-state value.
One prior art switching power supply has an integrated regulator circuit with a combined feedback and power supply on a single input pin. The integrated regulator circuit has separation circuitry inside the chip to split the feedback and power supply signals. The combination of feedback and power supply signals on a single pin allows the integrated regulator to be implemented with fewer pins. In many applications however, there is a sufficient number of available pins that it is not necessary to combine feedback and power supply on a single pin. The design of the integrated regulated circuit is simplified where feedback and power supply are brought in on separate pins. In applications where feedback and power supply are brought in on a single pin the separation circuitry adds unnecessary complexity in the integrated circuit without a corresponding need or benefit.
Thus, a need exists for an integrated regulator circuit which uses completely separate operating voltage and feedback input pins to eliminate the need for complex separation circuitry. The circuit must also limit operating voltage extremes so low voltage circuitry can be used within the integrated regulator circuit.